(1) Field of the Invention
The present invention relates to a solid-state image sensing device and a camera, having a source-follower amplifier, and particularly relates to improvement of an output amplifier that converts signal charge generated by a photoelectric conversion unit into voltage and outputs the converted voltage.
(2) Description of the Related Art
Taking a charge coupled device (CCD) type solid state imaging device as an example, a solid state imaging device includes a photoelectric conversion unit, a charge transfer unit, and an output amplifier which converts signal charge into voltage signal, then amplifies and outputs the voltage signal. A floating diffusion amplifier is widely used as an output amplifier. The floating diffusion amplifier is structured by a floating diffusion layer which functions as a charge detection capacitor for converting signal charge to voltage, and a source-follower amplifier which is connected to the floating diffusion layer. In order to realize high output voltage and high signal-to-noise (SN) ratio in a solid-state image sensing device, further improvement of gain in the output amplifier is required.
FIG. 1 shows a circuit which is widely known for the conventional output amplifier technology. As shown in the diagram, the output amplifier is a 3-stage, source-follower amplifier, and each stage is structured by one metal-oxide semiconductor (MOS) type driver transistor and one MOS-type load transistor (for example, see: T. Ando, H. Komobuchi, “Basis of Solid-state Image Sensing Device”, Japan Science Publishing Association, 1999, p. 79). Furthermore, FIG. 2 shows the application of a current mirror circuit as an output circuit of an amplification-type solid state imaging apparatus disclosed in Japanese Laid-Open Patent Application No. 2000-278608 Publication. In addition, a semiconductor apparatus having a cascode FET in the output buffer (Japanese Laid-Open Patent Application No. 06-232656 Publication) is also widely known.
The following formula gives the small-signal voltage gain “G” of a source-follower amplifier:G=gm/(gm+gds+gmb+gl)  (Formula 1)
Here, “gm” is the mutual conductance of the driver transistor, “gds” is the drain conductance of the driver transistor, “gmb” is the backgate conductance of the driver transistor, and “gl” is the conductance of the load circuit.
Although the load circuit conductance gl serves as the drain conductance of the load transistor in the conventional output amplifier, it cannot be adequately reduced in comparison with the mutual conductance of the driver transistor gm, and is one cause for the deterioration of the small-signal voltage gain G. In particular, as there are many cases where the power source voltage of the output amplifier is set in common with the voltage of the charge reading pulse from a photodiode to the CCD, and a relatively high voltage of 10 to 15V is set in the CCD-type solid state imaging device, a high voltage of about 10V is also applied between the source and drain of the load transistor, and reduction of drain conductance is difficult.
Furthermore, when the source-drain voltage of the load transistor becomes a high voltage, there is a possibility of a hot-carrier being generated from the increase in the electrical field in the vicinity of the drain within the channel of the load transistor, and the problem of light emission can occur in connection with the impact ionization of the carrier. When a light ray of the emitted light reaches the imaging unit of the imaging device, it becomes a false signal of the output signal from the imaging device, and causes the deterioration of imaging characteristics.